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Old 17th April 2003, 12:21 AM   #1
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Default Need advice on replacing caps in amp P/S

Hello! I'm subscribing from Tokyo. This is my first post.

I'm inexperienced in amplifier building and repair, but I need to replace a rather unusual electrolytic capacitor in the power supply of a 28W+28W Class A transistor amplifier made by Onkyo in 1976. The component to be replaced is a single large (75 x 110 mm) component but is functionally the equivalent of two 15000uF capacitors connected in series, with the center point connected to the bus earth line. This component has three connectors, "Plus," "E," and "Minus"; it is marked with the values "15000uF + 15000uF 80WV."

Since I have enough room and since I found a cheap source of computergrade capacitors, I am thinking of replacing this component with an array of six identical 12000uF 80V capacitors. (For example, creating two 36,000uF parallel chains and then joining the two chains in series would be one possibility.) Other configurations are also possible, and it would also be possible to use only four of these capacitors rather than six.

Here are my questions:
1) What is the best strategy to take here? Four capacitors? Six? What configuration?
2) Do I need to use balancing resistors? If so, how should I use them?

Thanks for your help!

CDWitmer

P.S. I have scanned photos of the amp, the capacitor in question, and also schematics if anyone feels these are necessary to answer the question. (Obviously I didn't want to post these directly to the board.)
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Old 17th April 2003, 12:47 AM   #2
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Hiya CD and welcome to the forums...I`m new here too.....back to your question.....you can replace those caps with single ones as you stated with as many as you want in parallel...just make sure they have the same working voltage...and dont use any resistors....just maintain the polarity.


Cheers!! The DIRT®
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Old 17th April 2003, 02:39 AM   #3
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Default If I connect only in parallel, how do I earth?

Joe Dirt, thanks for your reply. Perhaps connecting in parallel w/o resistors is all I need to do; however, I'm curious as to what I need to do about grounding the capacitors . . . the original component has three connectors; the one in the middle is "Earth." If I have all my caps connected in parallel only, where do I put the connection to earth?

I decided to post a pic of the amp and part of the schematic after all. (I hope this works!) At the bottom of the big caps in the center of the amp, the three connectors can be readily seen. The corresponding parts of the schematic are where the letters are rotated at a 90 degree angle from all the other lettering.

Now, assuming I *shouldn't* connect only in parallel. but need to have a series connection to facilitate earthing as in the schematic, if I have two parallel capacitor "chains" connected in series, can I still get away without resistors? I have read that if only two caps are connected in series, a balancing resistor isn't needed. Can I consider each of the parallel "chains" as a single big cap? Or am I asking for trouble if I hook up capacitors this way without using any balancing resistors?

Thanks very much to all.
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Old 17th April 2003, 08:41 AM   #4
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Default More of schematic

Hi CD,

would it be possible to post more of this schematic. Need to see where the 'earth' point of your capacitor pair connects to. From what you've already posted it connects to a thicker schematic line and then off the top of the page, though with a line heading back to the ac section of the diagram on the right. Need to determine (at least) if that point is going back to a tap on the transformer.

James
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Old 17th April 2003, 09:41 AM   #5
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Quote:
Can I consider each of the parallel "chains" as a single big cap?
If you connect N caps with a capacitiy of X Farads in parallel then you get a thing that behaves as a single big cap with N*X Farads.
Balancing resistors are needed for series connections of caps.

I think your amp has two separate power supplies with center tapped transformers. So you will most probably not need any resistors.

The amp seems to be a nice and tidy construction and worth the hassle of a refit.

Regards

Charles
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Old 17th April 2003, 12:04 PM   #6
jcarr is offline jcarr  United States
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CDWitmer:

Your amplifier looks very nice. The Class A operation and monaural power supplies suggest that it belonged to Onkyo's premium series. That being the case, I wouldn't be surprised if it includes other technical features that are quite interesting, even by today's standards. It may even be an early MOSFET or vertical power FET design.

What is the model name?

BTW, electrolytics tend to fail over time, and in an elevated thermal environment (such as can be expected in a Class-A power amp), they can leak electrolytic fluid onto the pcb, which will eventually eat away the copper traces and cause a melt-down. A complete replacement of all electrolytics may not be a bad idea if you want your Onkyo to remain operative for many more years. Be especially wary of electrolytic capacitors that have a slight "bulged" surface at the top of the metal cans.

regards, jonathan carr (also in Tokyo)
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Old 17th April 2003, 12:31 PM   #7
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CD...that is a very nice amp!!!!!....I would almost leave the cover off or replace it with a glass cover.......as far as your question...there are two primary supplies I see....each negative (ground) supply are common to each other...so that being said you could do a bank for each section tieing the the negative to the common ground...your not connecting in series here but in parallel to each rectifier stage...I dont know if I`m explaining this right...if you dont understand I will draw up a quick schematic.


Cheers!!The DIRT®
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Old 17th April 2003, 01:04 PM   #8
PassFan is offline PassFan  United States
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Are you sure those caps are bad? Did you try to repower them? What it looks like to me is a +/0/- all in one can. I've never seen that before, although that's not surprising as I've only been about this for 8 or so years. That is a nice looking amp. Love those transformers. Keep at it, as there's enough people here to get it to work.
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Old 17th April 2003, 01:07 PM   #9
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passfan...they were common years ago to save room and parasitics in the circuit
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Old 17th April 2003, 01:21 PM   #10
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you are perfectly right Passfan....I just assume that he made those tests....and its great that you have my back Bro...we need all the input from everyone here...we all have different strengths and weaknesses in electronics

Cheers!!The DIRT®
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